So-called ADSL (asymmetric digital subscriber line) communications systems are known in the form of multicarrier modulation (MCM) communications systems which are conveniently implemented as discrete multitone (DMT) systems using Discrete Fourier Transform (DFT) and inverse DFT (IDFT) techniques. American National Standard for Telecommunications document T1.413 describes a particular form of such an ADSL system, and the invention is accordingly described below in the context of such an ADSL system. However, it can be appreciated that the invention is applicable to other forms of MCM communications systems, whether or not they are DMT systems and whether or not they provide asymmetric communications.
A T1.413 ADSL system uses a superframe structure in which each superframe is composed of 68 data frames, numbered 0 to 67, which are encoded and modulated into DMT symbols, followed by a synchronization symbol (also referred to as a synchronization frame), which is inserted by the modulator to establish superframe boundaries. From a user data perspective the DMT symbol period is 0.25 ms corresponding to a DMT symbol rate of 4000 symbols per second, but in order to allow for the insertion of the synchronization symbol the transmitted DMT symbol rate is increased by a factor of 69/68; the superframe period is 17 ms.
In each data frame, buffered data is allocated to the different tone or carrier subchannels in a manner which is dependent upon the signal-to-noise ratios (SNRs) of the subchannels, typically so that bit error rates (BERs) of the subchannels, as monitored at the receiver, are substantially equal. As a result, within each data frame different sub-channels carry different numbers of bits. With an appropriate allocation of bits and transmit powers to the different subchannels, such a system can provide a desirable performance.
Over a period of time during normal operation of such an ADSL system, for example with changes in temperature, traffic on adjacent transmission channels, and other sources of interference, the SNRs of the different subchannels will vary, so that it is desirable to update the allocations of bits to subchannels during operation of the system. Peter S. Chow et al. U.S. Pat. No. 5,479,447 issued Dec. 26, 1995, entitled “Method And Apparatus For Adaptive, Variable Bandwidth, High-Speed Data Transmission Of A Multicarrier Signal Over Digital Subscriber Lines”, discloses a procedure for initializing bit allocations in an MCM system, and proposes an adaptive updating procedure for such allocations.
It can be appreciated that the bit allocations which are used by the transmitter and the receiver must be matched, and that changes of these bit allocations must be made at the same time (i.e. for the same superframe) in the transmitter and the receiver in a manner which is not subject to errors. Otherwise, the BER can be seriously impaired, and there may be a resulting need to fully re-initialize the system: a very undesirable consequence, especially considering the time (of the order of 10 seconds) required for full initialization.
To avoid errors in the bit allocation updating procedure, referred to briefly as bit swapping but more accurately relating to an updating of bit allocations and signal energies, or transmit gains, in what is referred to as a B&G (bits and gains) table maintained in each of the transmitter and the receiver) bit swap request messages can be sent from the receiver to the transmitter using an ADSL overhead control channel (referred to as the aoc) which is part of the ADSL framing structure. Each aoc message is transmitted 5 consecutive times, to provide a high degree of reliability, and the message can also be subject to forward error correction coding (FECC), codeword interleaving, and trellis coding of the system. In response to the bit swap message, the transmitter can send an acknowledge message on the aoc, and the bit allocation is subsequently performed at both the transmitter and the receiver.
Ronald R. Hunt et al. U.S. Pat. No. 5,400,322 issued Mar. 21, 1995, entitled “Updating Of Bit Allocations In A Multicarrier Modulation Transmission System”, discloses such a so-called bit swapping procedure, together with a numbering of DMT symbols for synchronizing bit allocation changes between the transmitter and receiver of an ADSL system. In addition, this patent recognizes that because the subchannels carry variable numbers of bits, the total transmission rate of the system is not fixed but can be increased (resulting in reduced SNR) or decreased (resulting in increased SNR) to meet particular requirements.
Such variation in transmission rate, now referred to as seamless rate adaptation (SRA), serves to adapt the data transmission rate during operation of the ADSL system in a so-called seamless manner, i.e. without errors in or interruption of the data transmission. As can be appreciated from the above description, SRA involves changing the number of bits modulated in a DMT symbol (i.e. one data frame), and changing the B&G tables accordingly, without modifying other parameters of the ADSL system such as the FECC, interleaving, and framing. It can be appreciated that error-free changes to the B&G tables in a manner that is synchronized between the transmitter and the receiver is as important for SRA as it is for conventional bit swapping, for similar reasons.
Whereas conventional bit swapping is initiated by the receiver, SRA can be initiated by either the receiver or the transmitter. In the former case the receiver sends an SRA request message to the transmitter, and in the latter case the transmitter sends an initial SRA request message to the receiver which can accept this request by sending an SRA request message to the transmitter. In each case the transmitter can grant the request by sending a signal referred to as SRA_GO to the receiver, after which the transmitter and receiver both use changed B&G tables accordingly.
In either conventional bit swapping or SRA, there remains a risk of errors causing a loss of synchronism between the B&G tables used in the transmitter and the receiver. For example, the acknowledge message or SRA_GO signal may be transmitted by the transmitter but may become corrupted due to a burst of errors and not recognized by the receiver. In this case, the transmitter will switch to a changed B&G table but the receiver will not, resulting in such a loss of synchronism.
In order to address such a problem, “The Essential Merit Of Bit-Swapping” by John M. Cioffi suggests that after transmitting a bit swap request message the receiver can monitor the incoming signal from the transmitter using both old and new B&G tables, and can use an FECC error flag to determine which of the tables is correct and should continue to be used. This proposal would be relatively inconvenient and costly to implement, because the FECC operates on data bytes rather than on the subchannels or tones and their allocated bits so that translation between these would be necessary, and is further complicated by interleaving.
It has been proposed in “Proposed working text for Seamless Rate Adaptation (SRA) for G.dmt.bis and G.lite.bis”, Aware, Inc., ITU—Telecommunication Standardization Sector, Study Group 15, Temporary Document BA-087, 19-23 Jun. 2000, to implement the SRA GO signal as an inverted synchronization symbol (180 degree phase shift). It will be recalled from the description above that the synchronization symbol, which comprises a known pseudo-random data sequence or pattern, is inserted after every 68 data frames (DMT symbols) to establish superframe boundaries.
It is observed here that during initialization of a T1.413 ADSL system, various signals are used including signals which can be generically referred to as REVERB and SEGUE. The REVERB signals use the same pseudo-random data pattern as is used by the synchronization symbols. Except for a pilot tone, the SEGUE signals comprise a tone-by-tone 180 degree phase reversal of the REVERB signals (i.e. +maps to −, and −maps to +, for each signal point of the 4-QAM constellation that is used). The pilot tone is a specific subchannel or tone onto which the data modulated is a constant {0,0}, generating the {+,+} 4-QAM constellation point, used to facilitate resolution of sample timing. Thus except for this pilot tone, the SEGUE signal is an inversion of the REVERB signal, corresponding to the synchronization symbol. As used herein inversion of the synchronization symbol includes such a tone-by-tone 180 degree phase reversal for all or substantially all of the tones, whether or not this includes a 180 degree phase reversal for any specific tones used for special purposes, such as the pilot tone. Terms such as inversion and inverted as used herein are to be understood accordingly.
However, it has been recognized that with the above proposal a burst of noise might conceivably cause a normal synchronization symbol to be interpreted as an inverted synchronization symbol, in which case the receiver could prematurely implement a change in its B&G table. To reduce this risk, it has been proposed in “G.gen: SRA features and messages”, Alcatel, ITU—Telecommunication Standardization Sector, Study Group 15, Temporary Document HC-049, 31 Jul.-4, Aug. 2000, to send a superframe count with the SWA swap request message to identify where the SRA_GO signal is expected, and for the receiver to monitor only the particular expected synchronization symbol for inversion to constitute the SRA_GO signal. Even in this case, there is a possibility that the actually inverted synchronization symbol constituting the SRA_GO signal may be corrupted by noise so that it is wrongly interpreted as a non-inverted synchronization symbol, in which case again the transmitter would implement the relevant SRA changes and the receiver would not.
In the various error situations discussed above, recovery techniques can be used for example as described by John M. Cioffi as discussed above, or re-initialization or retraining of the system, but these techniques are not particularly desirable because of their adverse impacts on the architecture and complexity of the system and/or data transmission.
A need exists, therefore, to provide an improved method of notifying a receiver of bit allocation changes in a multicarrier modulation communications system.